The primate lentiviruses all encode for a Vif protein that has been shown to interact with members of the apolipoprotein mRNA-editing, catalytic polypeptide-like 3 (APOBEC3) superfamily of proteins. The APOBEC3 proteins are cytidine deaminases that are thought to play an important role in innate anti-viral restriction. Humans and macaques both encode for seven APOBEC3 genes (A3A, A3B, A3C, A3D, A3F, A3G, and A3H). The APOBEC3 proteins can be broadly divided into the single deaminase domain (A3A, A3C, and A3H) and double deaminase domain proteins (A3B, A3D, A3F, and A3G). The thymus is essential to CD4+ T cell homeostasis and is the major source of nave CD4+ T cells throughout life. HIV-1 infection of humans can lead to the inhibition of proliferation of immature thymocytes and/or can directly infect CD4+ thymocytes leading to impaired production of CD4+ T cells. Thus, an imbalance between production and destruction of peripheral CD4+ T cells likely leads to their progressive decline over time and eventually to AIDS. The thymus is particularly important in pediatric patients who depend heavily on their thymus for generation of new CD4+ T cells. To date no information is available on the expression of the A3 genes/proteins in cells of the developing thymus. We present preliminary data that indicate that A3G/A3F proteins are not expressed in the double negative (DN: CD4-CD8-) or double positive (DP:CD4+CD8+) thymocytes but are expressed in single positive (SP CD4+) cells of the human thymus. This indicates that the expression of this important virus restriction factor is likely developmentally regulated. In Specific Aim 1, we propose to perform a comprehensive examination to determine the stage of thymocyte development when A3G/F gene (and other A3 genes) expression becomes activated. In Specific Aim 2, we propose to determine if the stage of A3 gene expression correlates with the ability to restrict HIV-1 and HIV?vif viruses and determine if known inducers of A3 gene expression will stimulate expression. The proposed studies will provide novel insight into the expression of A3 genes during development, viral persistence in the thymus, and in the development of antiviral drugs targeting Vif/A3 interactions.